Method of isostatically pressing metal powder into desired metal shapes

ABSTRACT

ISOSTATIC PRESSING OF META POWDER IS CARRIED OUT IN AN IMPROVED MANNER BY UTILIZING A SPECIALLY FORMED TOOLING ENCLOSURE BODY MADE OF A RELATIVELY HARD PLASTIC MATERIAL WHICH TAKES THE PLACE OF CONVENTIONALLY EMPLOYED SOFT RUBBER. THE RELATIVELY HARD PLASTIC MATERIAL, WHILE HELD IN CONTACT WITH A MOLDING PATTERN IS THERMO-FORMED UNDER VACUUM AND ITS THICKNESS IS CONTROLLED TO PROVIDE RELATIVELY STIFF WALL PORTIONS CAPABLE OF RECEVING A QUANTITY OF METAL POWDER AND SUSTAINING ITS WEIGHT WITHOUT DISTORTION OR APPRECIABLE DIMENSIONAL CHANGE DURING PROCESSING. HYDROSTATIC PRESSURE EXERTED THROUGH THE PLASTIC TOOLING ENCLOSURE PRODUCED A METAL SHAPE MADE TO EXACTING SPECIFICATIONS AND CHARACTERIZED BY HIGH QUALITY SURFACE FINISHING.   D R A W I N G

June 26, 1973 POWDER INTO DESIRED METAL SHAPES 5 Sheets-Sheet 1 Filed Nov. 4, 1971 D. 22422, 3 WWW flifiibz'aeg 2 kmkb p bofikb \MRS N \MRS o m k% mm oruka o mull h" R NK? 0' Q ofdku) A or"??? 4 Q k A Y \Wlxb m 0 km June 26, 1973 0. 0. ALLEN 3,741,755

METHOD OF LSOSTA'IICALLY IRESSING METAL POWDER INTO DESIRED METAL SHAPES Filed Nov 4, 1971 5 Sheets-Sheet June 26, 1973 ALLEN 3,741,755

METHOD OF ISOSTATICALLY PRESSING METAL POWDER INTO DESIRED METAL SHAPES Filed Nov. 4, 1971 5 Sheets-Sheet 5 Iawezzior DMDJHW,

dliiba raey June 26, 1973 D. D. A EN 3,741,755

METHOD OF ISOST CA RESSING METAL DER INTO IRED TAL SHAPES POW Filed Nov. 4, 1971 Sheets-Sheet 4 June 26, 1973 0. D. ALLEN 3,741,755

METHOD OF ISOSTATICALLY PRESSING METAL POWDER INTO DESIRED METAL SHAPES Filed Nov. 4, 1971 5 Sheets-Sheet 5 v 4 l as I I 40 United States Patent US. Cl. 75-214 14 Claims ABSTRACT OF THE DISCLOSURE Isostatic pressing of metal powder is carried out in an improved manner by utilizing a specially formed tooling enclosure body made of a relatively hard plastic material which takes the place of conventionally employed soft rubber. The relatively hard plastic material, while held in contact with a molding pattern is thermo-formed under vacuum and its thickness is controlled to provide relatively stiff wall portions capable of receiving a quantity of metal powder and sustaining its weight without distortion or appreciable dimensional change during processing. Hydrostatic pressure exerted through the plastic tooling enclosure produces a metal shape made to exacting specifications and characterized by high quality surface finishmg.

This invention relates to processing metal powder to form metal shapes of various configurations and more particularly the invention is concerned with an improved method for supporting a shaped mass of metal particles and subjecting the shaped mass to isostatic pressing to produce a solidified body.

In conventional isostatic pressing of metal powders, it is customary to introduce a quantity of metal powder into a soft rubber tooling enclosure body which is capable of being compressed under hydrostatic pressure to provide a solidified mass. Thereafter, the mass is subjected to heat to carry out sintering. This conventional isostatic pressing method, as is well-known to those skilled in the art, is subject to problems which arise out of the use of the soft rubber employed to form the tooling enclosure body. Thus in filling the soft rubber tooling with metal powder, its wall portions have insufiicient stiffness to sustain the weight of the mass and to resist deformation and this may result in an undesirable change in the metal shape. If the walls of the soft rubber are made sufliciently thick to resist deformation, hydrostatic pressure is not adequately transmitted to the powder during pressing and a faulty metal shape frequently results. External rigid supporting means may be provided during filling of the powder, but the support must be removed during pressing, at which point, the filled tooling may again be subject to distortion.

Still another problem arises in the use of soft rubber as a tooling means. It should be understood that the ultimate aim of isostatic pressing is to make metal shapes in a way to eliminate costly finishing processes such as machining. This is thwarted by the soft rubber because its inner tooling surface may be subject to particle indentation, and as a consequence, the surface of the pressed shape tends to present a rough granular appearance.

It is a chief object of the invention to cope with the difiiculties noted above and inherent in the use of a soft rubber for forming a tooling enclosure body. It is a further object of the invention to devise a method of supporting a mass of powder particles in an enclosure which is capable of sustaining the weight of the mass without significant dimensional change or deviation from a required tooling shape, and which will nonetheless transmit hydrostatic pressure in an acceptable manner. It is still a further object of the invention to provide a tooling Patented June 26, 1973 "ice enclosure body characterized by inner surface areas of mechanical hardness or strength which will withstand particle indentation and which can be utilized to produce an isostatically pressed shape with a high quality surface which avoids subsequent finishing.

With the foregoing objectives in mind, I have conceived of an improved method of processing metal powder to produce isostatically pressed metal shapes. My improved method is based upon substituting for soft rubber tooling a plastic enclosure body whose mechanical strength properties more nearly approach those of some metals while retaining good pressure-transfer characteristics. To this end, I have discovered that I may successfully use certain plastic materials having a unique combination of mechanical strength properties.

I have determined that certain plastic materials having a relatively high hardness characteristic, for example, of the order of a Rockwell hardness R-IOO, combined with a relatively low elastic modulus characteristic of the order of several hundred thousand p.s.i. can be made in relatively flexible form if utilized in a suitable thickness, for example, in the order of .020 inch. I have further found that in this flexible form with properly controlled thickness, the plastic materials have mechanical strength properties sufiiciently like that of metals to become formed into a tooling enclosure body capable of retaining its shape under the weight of a mass of metal powder of a considerable range of weights.

I also find that by properly combining hardness and elastic modulus with thickness control, the plastic tooling enclosure body can function to transmit pressure in a satisfactory manner and its inner surface finish is superior to that of a machined metal surface. Thus there may be produced in a pressed metal shape a high grade surface which far su1passes the quality obtained in soft rubber tooling.

As an example of plastic materials having the combination of high hardness characteristic and low elastic modulus, which I have found can be used successfully, there may be cited such compounds as Luran S, a sheet material manufactured and sold by Koro Plastics Co., Inc., having a place of business in Hudson, Mass, and consisting of an acrylic elastomer designated as an acrylonitrile styrene acrylic elastorner. Other materials such as Plexiglas and the like, with suitably controlled characteristics, may also be employed.

In order to utilize plastic materials of the type specified above formed in a workable thickness to constitute walls of a tooling enclosure body, I have also devised a novel method of thermal forming under vacuum by means of which a sheet of the plastic material can be very accurately formed to produce the shape of the molding pattern while providing for a controlled thickness factor which will suitably transmit hydrostatic pressure.

The nature of the invention and its other objects and novel features will be more fully understood and appreciated from the following description of a preferred embodiment of the invention selected for purposes of illustration and shown in the accompanying drawings, in which:

FIG. 1 is a diagrammatic view of the method of isostatic pressing of the invention;

FIG. 2 is a view showing a molding pattern and a sheet of plastic material located immediately above;

FIG. 3 is a perspective view illustrating a step of thermal forming the sheet of plastic material of FIG. 2 and conforming it to the shape of the pattern under vacuum;

FIG. 4 is a perspective view illustrating a vacuumformed tooling enclosure body produced by the method i1- lustrated in FIG. 3;

FIG. 5 is a cross-sectional view of the tooling enclosure;

FIG. 6 is another cross-sectional view of a tooling enclosure body filled with metal powder;

FIG. 7 is a cross-sectional view illustrating the tooling enclosure in a completely filled and capped condition;

FIG. 8 is a cross-sectional view of isostatic pressing ap paratus illustrating the metal powder filled tooling enclocure immersed in a fluid member together with means for exerting hydrostatic pressure therethrough;

FIG. 9 is a detail view of the sealed enclosure showing portions thereof partly stripped away;

FIGS. 10, 11 and 12 are modified forms of tooling enclosure bodies.

The method of the invention in its broadest aspect includes two basically new steps which are employed in conjunction with the conventional operation of isostatically pressing a quantity of metal into a solidified mass and then sintering the mass. One of these steps consists in introducing into a tooling enclosure body, specially formed for hardness characteristic, a known weight of metal required for a desired metal shape and simultaneously sustaining the weight of the metal powder without significant distortion or dimensional change in the wall portions of the enclosure body. A second important step consists in sealably containing the enclosure body and powder in a fluid medium and exerting hydrostatic pressure which is not only operative to compact the metal powder, but is also transmitted through the tooling wall portion which resists particle indentation. Both of these two basically new steps are carried out by employing a specially formed tooling enclosure made with a novel construction of hardness and flexing properties.

It will be understood that a first procedure in carrying out the method of the invention is to provide a known quantity of metal particles such as aluminum, iron, copper and the like, suitable for making a desired metal shape. A second procedure is to prepare a tooling enclosure body which is pre-formed to the shape of a molding pattern of the desired metal shape.

As noted above, I have discovered that certain metallike plastics having a relatively high hardness characteristic may be used to form the tooling enclosure body, and an important feature of the invention consists in preforming a special sheet of plastic material of a relatively high Rockwell hardness characteristic combined with a relatively low elastic modulus characteristic, and controlling the pre-forming of the sheet so that it has proper weight-sustaining strength combined with suitable pressure-transmitting capability.

As illustrative of a typical instance of making a metal shape in accordance with the method of the invention, I have indicated in the drawings a series of steps for producing a pressed metal shape having a simplified form consisting of a bar of metal of a desired length, Width and thickness obtainable from a quantity of metal powder weighing approximately one pound.

A molding pattern with suitable tolerances is first provided to correspond to the desired shape and size of the bar of metal to be made. Utilizing this molding pattern, I then prepare a tooling enclosure body which is formed from a special thermo-plastic sheet material consisting of a compound having a relatively high Rockwell hardness characteristic combined with a relatively low elastic modulus characteristic. As illustrative of such a sheet material, there may be cited a plastic of the class commonly referred to as an acrylic resin of acrylonitrile, styrene, also sold under the trade name Luran S.

In pre-forming this special thermo-plastic sheet material, I may control the thickness of the sheet. For example, with a one-pound sample of metal powder, the thickness may be held within a range of from about .020 inch to about .022 inch. It should be understood that in varying the method of the invention to produce dilferent required shapes, larger or smaller quantities of metal powder are utilized, and the thickness and hardness of the tooling body wall portions may be controlled in accordance with the change in weight to obtain a proper balance between weight-sustaining capability and pressuretransmitting properties. In pre-forming the tooling enclosure body in the controlled manner described, the sheet material specified is utilized in a suitable size for completely overlying the molding pattern and may be arranged in superimposed relationship on the pattern as suggested diagrammatically in FIG. 1 at Step A. In FIG. 2 of the drawings, the sheet material is indicated by numeral 2 and the molding pattern is denoted by numeral 4, occurring in separated relationship to the sheet material.

As one desirable means of pre-forming the sheet material in accordance with the invention, I subject the sheet to heating while superimposed on the pattern and thus cause it to assume a plastic state. While in this plastic state, I subject the sheet to a vacuum exerted by removing air from the underside of the sheet. These two steps are noted as Steps B and C in FIG. 1, and have also been suggested diagrammatically in FIG. 3. As illustrative of apparatus for carrying out the combined heating and vacuumizing procedure noted, I may employ a machine such as the Sign Smith Vacuum-Forming Machine, Model 2630T, manufactured by Nova Mfg. Co., Inc., Bessemer, Ala.

It will be understood that the tooling sheet and the pattern are placed in this vacuum-forming machine indicated at 6 in FIG. 3. The machine is not shown in the drawings in detail as it is well-known to those skilled in the art. The lid of the machine is then closed to make the forming chamber airtight. The temperature of the plastic is thereafter raised, for example, to a temperature of 290 F. at which point a vacuum of approximately 7 inches of mercury is exerted through the evacuating apertures 6a. This pulls the thermally softened plastic against the pattern for approximately 30 seconds, for example, causing the plastic to take the shape of the pattern and provide the tooling enclosure body 8, as shown in FIG. 4.

In the typical instance described, I find that fine detail is realized by such a tooling element to an extent such that the tooling material will reproduce circular openings of .001 inch in diameter. The resulting tooling enclosure body, thus pre-formed, presents relatively stitf wall portions having a thickness varying from .010 to .020 inch, and these relatively stiff wall portions have a Rockwell hardness, for example, of R102, a yield strength of 6300 p.s.i. and a modulus of elasticity of 330,000 p.s.i., and are capable of sustaining a weight of one pound, for example, without distortion or dimensional change.

I find that it is essential to maintain the thickness of the tooling enclosure body 8, particularly at the wall portions 8a, which are decreased slightly in thickness by reason of the vacuum force, at a value of not less than .010 inch in order to provide resistance to tearing or separating.

The tooling enclosure body, when thus completed, is removed from the vacuum-forming machine as suggested at Step D, and then located on a vibrating holder at' Step E. In some cases, a tooling body may be made in halves, which are thereafter cemented together with cement, and in such case, a fill opening for receiving a metal powder is provided. However, in the simplified arrange ment shown in FIGS. 2-7 inclusive, the tooling enclosure body is made in full size with an open side to be closed by a cover after having been filled with powder.

In carrying out the operation of filling the enclosure body 8 with the metal powder, I may provide a fill opening at 8b in one side of the member 8 and the open side is supported against a cover element 10 as suggested in FIG. 6. With the tooling enclosure in this position, predetermined weight of aluminum metal powder of one pound, for example, having particle size of minus 325 mesh, is then introduced through the fill opening while the enclosure body is being vibrated at a frequency of 60 cycles per second and an amplitude of .020 inch. The metal powder particles, as they enter the enclosure, are gradually compacted into a mass, as suggested in Step E and also in FIG. 6.

This provides for completely filling the tooling, as indicated diagrammatically at Steps F and G in FIG. 1, and and also in FIG. 6. It is again pointed out that in accordance with the invention, this operation is accomplished with the relatively stiff wall portions 8a sustaining the weight of the metal powder without significant distortion or dimensional change.

As soon as the filling operation is completed, the filled opening is sealed, as indicated at Step H in FIG. 1 and also in FIG. 7. The sealed tooling enclosure, together with the quantity of metal powder contained therein, is then enclosed in a flexible protective casing 14, as shown in FIG. 8. This assembly is then placed in a rigid container 16 which is, in turn, immersed in a hydrostatic fluid 18 received within a pressure vessel 20.

The hydrostatic fluid is free to pass through openings 22 in the top of member 16 and surrounds the casing 14. A hydrostatic pumping system 24 exerts a hydrostatic pressure, for example of 60,000 p.s.i. through the fluid 18, and this pressure is transmitted through the casing 14 and the wall portion 8a of enclosure 8 to compact the powder and form a solidified mass. It is pointed out that this transfer of pressure is accomplished through the relatively stiff wall portions made as specified so that no particle indentation takes place and the surface of the solidified mass takes on a finish corresponding to that of the inner surfaces of the wall portions 8a. Locating the assembly in the hydrostatic medium 18 and exerting pressure to solidify the mass of particles is illustrated in FIG. 1 by Steps I and J.

Thereafter, the assembly is removed from the pressure vessel and the tooling enclosure body is stripped away from the pressed metal block B as suggested in FIG. 9 and also at Step K in FIG. 1. The pressed metal shape B is then ready for the usual sintering operation carried out in a conventional heating furnace 26, as indicated in FIG. 10, and also at Step L in FIG. 1.

As noted above, the weight of the metal sample may vary in making larger and smaller shapes, and as already noted, the thickness of the enclosure body may be controlled in accordance with such changes in weight as may be encountered. As illustrative of typical changes in weight and the related changes in tooling thickness, there may be cited, for example, in the case of a part having a weight of 5 lbs. a tooling enclosure having a thickness of from .035 to .045 inch, and in the case of a part having a weight of lbs., the thickness of the tooling may be from .050 inch to .060 inch.

In order to prevent bending of the tooling and powder when pressing long slender shapes, I may desire to employ a metal backing strip attached to the plastic tooling either by a mechanical fastening device or by a layer of cement such as a contact cement. The surface of the backer must conform to the shape of the tooling at the area of contact. The thickness of the backer required, for example, for an applied hydrostatic pressure of 60,000 p.s.i., may be approximately of an inch for aluminum, and in the case of steel% of an inch. Such a backer strip may possess simple surface shapes such as planar or semicylindrical. The backer strip need be attached on only one surface of the tooling and gives general support to the whole tooling structure during pressing to prevent bending of the pressed part. A typical backer strip 28 has been indicated in FIG. 8 and also suggested by Step M of FIG. 1.

Where the shapes of the tooling, and therefore, the finished external surfaces of the pressed pieces, are too complex to be mated with an external backer strip, a simply-shaped core 30 may be embodied in a pressed metal shape 31, contained in a tooling 'body 32, as suggested in FIG. 12. The core 30 may be either a completely solid wrought or cast shape or a pressed and sintered shape with approximately 5% porosity. The core may be the same chemical composition as the powder, and its dimensions may be approximately of those of the tooling. The core may remain an integral part of the shape during pressing and may be bonded strongly to the surrounding metal powder during sintering. For parts having a more nearly equi-dimensional shape, i.e. length/diameter equal 2 or less, support is achieved through resistance of the plastic tooling between plates of a metal picture frame 34, as illustrated in FIG. 11. The plates of the frame 34 may be held together mechanically by screws 36 with flaps of the tooling fitting between the plates of the frame, as indicated in FIG. 11.

I claim:

1. That improved method of making a metal shape from metal powder which comprises introducing a known quantity of metal powder into a pre-formed tooling enclosure body having relatively stiff wall portions and being constructed of plastic, non-metallic material such as an acrylonitrile styrene acrylic elastomer, said plastic material having a relatively high Rockwell hardness characteristic combined with a relatively low elastic modulus characteristic, simultaneously sustaining the weight of the metal powder without dimensional change in the wall portions, sealably supporting the tooling enclosure body and the quantity of metal powder within a fluid medium, and then exerting hydrostatic pressure through the fluid medium to compact the metal powder into a solidified mass.

2. That improved method of making a metal shape from metal powder which comprises introducing a quantity of metal powder into a tooling enclosure body having wall portions which sustain the weight of the metal powder without dimensional change and which is constructed of a plastic, non-metallic material such as an acrylonitrile styrene acrylic elastomer, said plastic material having a relatively high Rockwell hardness of the order of R-lOO combined with a relatively low elastic modulus characteristic of the order of several hundred thousand p.s.i., sealably supporting the tooling enclosure body and the quantity of metal powder Within a fluid medium and then exerting in the fluid medium a hydrostatic pressure of an intensity to compact the metal powder into a solidified mass having smooth finished outer surfaces.

3. That improved method of making a metal shape from metal powder which comprises introducing a quantity of metal powder into a tooling enclosure body to fill a predetermined volume, said body having wall portions which are formed with smooth finished inner surfaces, are of a predetermined thickness and being constructed of a plastic material such as an acrylonitrile styrene acrylic elastomer, said plastic material having a relatively high Rockwell hardness characteristic of the order of R- combined with a relatively low elastic modulus characteristic of the order of several hundred thousand p.s.i., sustaining the weight of the metal powder without dimensional change while the enclosure is being filled, sealably supporting the tooling enclosure body and the quantity of metal powder within a fluid medium, and then exerting in the fluid medium a hydrostatic pressure of an intensity which compacts the metal powder into a solidified metal shape while maintaining the smooth inner surfaces of the wall portions free from particle indentation.

4. That improved method according to claim 1 in which the thickness of the tooling enclosure body is controlled in accordance with the weight of the metal powder employed.

5. A method according to claim 3 in which the thickness of the tooling enclosure body is controlled by heating the plastic while in contact with a molding pattern of the desired shape and simultaneously subjecting inner surface portions of the plastic material to a vacuum.

6. In a method of processing a quantity of metal powder to produce a metal shape in which the said quantity of metal powder is subjected to isostatic pressing, the steps which include providing a molding pattern of the said metal shape, superimposing on the molding pattern a heatformable plastic, non-metallic sheet material having a relatively high Rockwell hardness characteristic combined with a relatively low elastic modulus characteristic, heating the sheet material to a plastic state, subjecting the plastic material to vacuum forces to conform said heated sheet material to the outer surface of the molding pattern and produce a substantially rigid tooling enclosure for reproducing the shape of the molding pattern, introducing into the tooling enclosure said quantity of metal powder, said tooling enclosure sustaining the weight of the metal powder without dimensional changes in wall portions thereof, sealably supporting the enclosure in a fluid medium and exerting a hydrostatic pressure in the fluid which is transmitted through the tooling enclosure to isostatically press the quantity of metal powder into the said desired metal shape.

7. A method according to claim 6 in which the tooling enclosure is subjected to vibratory motion during the period in which the metal powder is introduced.

8. A method according to claim 6 in which the isostatically pressed metal powder is removed from the tooling enclosure and subjected to a sintering operation.

9. A method according to claim 6 in which the tooling enclosure body is supported during the step of filling with metal powder by means of a rigid backing element.

.0. A method according to claim 6 in which the tooling enclosure body is supported in a frame structure including four sides thereof.

11. A method according to claim 6 in which the tooling enclosure body includes a core element.

"12. In a method of processing a quantity of metal powder to produce a metal shape in which the said quantity of metal powder is subjected to isostatic pressing, the steps which include providing a molding pattern of the said metal shape, superimposing on the molding pattern a heat-formable plastic, non-metallic sheet material having a relatively high Rockwell hardness characteristic combined with a relatively low elastic modulus characteristic, heating the sheet material to a plastic state, subjecting the plastic sheet material to vacuum forces to conform said sheet material to the outer surface of the molding pattern for reproducing the shape of the molding pattern, controlling the vacuum forces to maintain the sheet material with a predetermined thickness, introducing through a fill opening in the tooling enclosure said quantity of metal powder and maintaining the tooling enclosure without significant dimensional change during the period in which the metal powder substantially fills the tooling enclosure, sealably supporting the enclosure in a fluid medium and exerting hydrostatic pressure in the fluid medium to isostatically press the quantity of metal powder into the said desired metal shape.

13. A method according to claim 1 in which the plastic material consists of an acrylic elastomer designated as an acrylonitrile styrene acrylic elastomer, and the wall portions are of a predetermined thickness.

14. A method according to claim 6 in which the heatformable sheet material consists of Luran S.

References Cited UNITED STATES PATENTS 2,298,908 10/1942 Wentworth 29--420 2,878,140 3/1959 Barr 29-l82 X 3,158,474 1l/1964 Andersen et al. -214 3,418,112 12/1968 Zoran et -a1 75-214 X 3,447,230 6/1969 Bargainnier et al. 29421 X 3,559,271 2/ 1971 Nilsson 29420.5 3,622,313 11/1971 Havel 75-214 X CHARLES W. LANHAM, Primary Examiner D. C. REILEY III, Assistant Examiner US. Cl. X.R.

29-420, 421 R, 423, DIG. 31; 264-111 

